Laser pumping apparatus

ABSTRACT

A circuit is disclosed for generating high voltage, low frequency alternating pulses having a high harmonic content of high frequencies sufficient to energize a gas laser. Two separate conductive electrodes are attached to the exterior of a gas laser tube and are connected to receive the high voltage for exciting and sustaining the operation of the laser.

United States Patent 1 1 Snow et al. 5

[ 1 Jan. 2, 1973 [541 LASER PUMPING APPARATUS [75] Inventors: Kenneth A.Snow; Richard E. Vandewarker, both of Greece, NY.

[73] Assignee: Bausch & Lomb Incorporated,

Rochester, NY.

[22] Filed: Nov. 27, 1970 [21] Appl. No.: 93,459

Related U.S. Application Data [63] Continuation-impart of Ser. No.748,289, June 29,

[52] U.S. Cl ..331/94.5 [51] Int. Cl ..H0ls 3/09 [58] Field of Search...331/94.5; 315/235, 240, 241 S [5 6] References Cited UNITED STATESPATENTS Morse ..33 l/94.5

3,341,708 9/1967 Bilderback ..331/94.5 3,351,870 11/1967 Goldsmith eta1. ..33 l/94.5 3,387,227 6/1968 Mastrup et a1. ..331/945 3,430,1592/1969 Roeber ..331/94.5 3,515,938 6/1970 Morse ..331/94.5 3,626,32612/1971 Wuerker et a1 ..331/94.S 3,633,127 1/1972 Caristi ..331/94.5

Primary Examiner-William L. Sikes AttorneyFrank C. Parker et a1.

57 ABSTRACT A circuit is disclosed for generating high voltage, lowfrequency alternating pulses having a high harmonic content of highfrequencies sufficient to energize a gas laser. Two separate conductiveelectrodes are attached to the exterior of a gas laser tube and areconnected to receive the high voltage for exciting and sustaining theoperation of the laser.

9 Claims, 6 Drawing Figures PATENTEDJAN 2191a sum 1 or 3 tobxkmi KENNETHA. suow III wm 8 i" II d \1 mp llllll II1 llll mm Q em 9 mm 8 4 6E mu mmRICHARD E. VANDEWARKER ATTORNEY naonnu v IO PATENTEDJAn 2 ms 76 0.5mm 726 1Q :2? g 7 FIG. 3 4

SHEET 2 OF 3 o m I Q m KENNETH A. SNOW RICHARD E. VANDEWARKER INVENTORSnrronnsi FATENTEDJM 2 I975 AAA VVV'W KENNETH A. SNOW- RICHARD E.VANDEWARKER INVENT I TIM FIG.

LASER PUMPING APPARATUS CROSS-REFERENCES TO RELATED APPLICATIONS Thisapplication is a continuation-in-part of our copending application, Ser.No. 748,289, filed on July 29, 1968, and assigned to the same assigneeas the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to apparatus for pumping a gas laser.

2. Brief Description of the Prior Art Presently, laser systems aregenerally pumped or excited by flash tubes, radio frequency energy orvery high voltage direct current potentials. The flash tubes aregenerally used with solid state lasers while the radio frequency anddirect current sources are used with gas lasers.

In the case of the radio frequency pumping sources, a high poweroscillator circuit generates electromagnetic signals in the megacyclerange. The radio frequency signals are applied to separate conductivecollars that surround portions of the laser discharge tube. In the caseof the high voltage direct current (DC) power sources (in the order of1700 volts) the DC potential is generally applied across two electrodesinserted through the laser tube into the gas medium. Both the radiofrequency and high voltage sources are individually rather bulky,complicated and expensive, thereby increasing the minimum cost of thelaser systems. Furthermore, since the direct current supply is appliedto at least one electrode extending through the glass laser tube, theprice of the tube itself is higher due to the necessity for providingexcellent glass to metal seals and the need for minimizing electrodesputtering, in addition to other requirements. Also, both the directpower supply and radio frequency power supply sources consume aconsiderable amount of power in their operation. As a result, it isdifficult to design such lasers into portable systems since such powersources cannot be practically energized by battery sources for any greatlength of time. Accordingly,'it would be an advantage to provide lowcost means for pumping gas lasers without requiring high voltage directcurrent or radio frequency power source that is particularly adapted forportable use.

A scheme for producing a pulse type laser beam is disclosed in a US.Pat. No. 3,351,870, issued to .l. Goldsmith et al. and entitled PulsedGas Laser. l-Iere direct current pulses are applied between an externalelectrode and an internal electrode at a rate so that the time betweenpulses is sufficient to allow the excited levels in the laser to decayto the ground state before the next pulse is applied. The operation ofthe laser is based upon the fact that electrons are emitted from themetallic surface of the internal electrode due to the applied electricalpulse to form a free electron cloud into the gas which in turn causesthe lasing action. The pulse is a unidirectional, short square wavepulse that requires a substantially shorter rise time than pulse width.As previously set forth, this type of laser and pumping system has thedisadvantage of requiring an internal electrode. Furthermore, the pulsesare stated to be unidirectional and require a particular shape, i.e.high rise time as compared to width.

SUMMARY or THE INVENTION It is therefore an object of this invention toprovide a new, low cost system for pumping a gas laser that does notrequire internal electrodes.

It is still a further object of this invention to provide an inexpensivesystem for pumping a gas laser that does not require internal electrodesand can provide a substantially continuous laser output.

The pumping system of the invention, includes a circuit for generatinglow frequency, high voltage alternating pulses that include a highharmonic content of high frequency signals in the megacycle range. Thecircuit, for example, may comprise an inductive inverter circuit. Twoseparate external conductors are positioned along different portions ofa gas laser. The circuit is connected to apply the high voltage pulsesbetween the two conductors and excite the laser into operation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an illustration of a laserpumping apparatus according to the principles of the present inventionincluding an inductive vibrator circuit;

FIG. 2 is a schematic diagram of the vibrator circuit of FIG. 1;

FIG. 3 is a schematic diagram of a semiconductor vibrator circuit thatcan be substituted for the inductive vibrator circuit of FIG. 1;

FIG. 4 is a schematic diagram of a modification of the circuit of FIG.3;

FIG. 5 is a graphic illustration of the high voltage pulses applied tothe laser by the apparatus of FIGS. 1 and 2; and

FIG. 6 is a graphic illustration of the high voltage pulses applied tothe laser by the apparatus of FIGS. 3 and 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The laser pumping circuit ofthe invention is illustrated as energizing a particulartype ofhelium-neon gas laser, however, it is to be understood that thisinvention will apply equally well to other types of gas lasers, such as,for example, helium-cadmium, cesium, neon-oxygen, helium-xenon,argon-oxygen. In FIG. 1 an elongated tube 10 composed of Pyrex glass ismounted on three spaced dielectric supports l2, l3 and 14. The tube 10includes a main cylindrical portion 15 with a double plane mirror 16 atone end and a curved mirror 18 at the opposite end, defining a resonantcavity for a gas laser. A cylindrical side arm 20 projects outwardlyfrom one end of the tube. portion 15 to thereafter extend generallyparallel to the tube portion 15. A reduced end 22 of the side arm 20carries a getter tivity at 6,328 A. Either, or both the mirror 18 andthe mirror 16 can be partly transmissive at the emitted wavelength.

A pair of external electrodes 26 and 28 are mounted along differentportions of the tube 10. In the particular example illustrated, theelectrode 26 surrounds the tube side arm 20, however, it could also bemounted along the main tube portion 15, as illustrated by the dottedlines 30. The electrodes 26 and 28 are connected to a vibrator inductiveinverter circuit 32 of the invention.

In the embodiment of FIG. 1, the inverter circuit 32 is illustrated as astandard Model T spark coil powered by a 6 volt battery 34. A schematicdiagram of the spark coil is illustrated in FIG. 2. The spark coilincludes a primary winding 40, a high turns ratio step-up secondarywinding 42, and a pair of contacts 44 electromagnetieally coupled to theprimary winding 40. An energizing direct current potential is appliedacross terminals 46 and 48 and the output voltage is taken fromterminals 48 and 50.

When the low voltage source is connected across the terminals 46 and 48,the primary winding 40 is energized until a sufficient field isgenerated to separate the contacts 44. The field collapses shortlythereafter and the contacts are again closed. The procedure is repeatedat a cyclic rate. Accordingly, the primary winding 40 is abruptlyenergized and de-energized wherein the magnetic field to secondarywinding 42 is also abruptly built-up and collapsed thereby developingvery high voltage spikes across the terminals 48 and 50,

as illustrated as the ringing type alternating pulses in FIG. 5. Theoutput wave forms comprise alternating voltage spikes that are ragged,include steep edges, and are rich in the Fourier spectrum. Byalternating, we mean that the polarity of the voltage across theelectrodes 26 and 28 reverses. Some of the high frequency components inthe alternating pulses are effective to excite the gas in the lasertube. It is believed that the frequencies which excite the gas in thelaser tube are in the order of the megacycle range. The output voltagehas a peak amplitude in the order of 10,000 volts. The contacts 44 ofthe spark coil are adjustable to provide a relatively low frequencychopping rate in the range of 80 to 400 cycles per second.

The Model T spark coil has the advantage that it can be readily poweredby any 6 volt direct current supply capable of delivering 2.5 3.0amperes of current. This allows a laser system of this sort to be highlyportable. The spark coil also has the advantage of being inexpensive,compact and a reliable source of power.

The circuit of FIG. 3 is a second embodiment of a laser pumping circuitof the invention energized from the 60 cycle line mains. The primarywinding 54 of an input power transfonner 52 is connected to theterminals of an AC power plug 56. The secondary winding 58 of thetransformer is connected through a pair of rectifiers 60 and 62 and acurrent limiting resistor 64 to a semiconductor inductive invertercircuit.

The semiconductor inductive inverter circuit includes a siliconcontrolled rectifier 66 (SCR) connected between the resistor 64 and thecenter tap of the secondary transformer winding 58. The gate electrodeof the controlled rectifier is connected to the rectifier 64 through aseries circuit including a resistor 68 and a trigger device 70, such asa neon bulb or a DIAC. The primary winding 72 of a step-up outputtransformer 74 is connected in series with a capacitor 76 across theanode and cathode electrodes of the controlled rectifier 66.Thesecondary winding 78 of the output transformer is connected to theelectrodes 26 and 2 8 of the laser tube 10. The transformer 74 by way ofexample can be a conventional automobile ignition transformer or coil.

The diodes 60 and 62 rectify the input 60 cycle voltage to provide 120cycle rectified pulses to energize the semiconductor inverter circuit.The capacitor 76 is initially charged through the primary winding 72until the rectified voltage is of sufficient amplitude to ignite ortrigger the neon lamp 70. At that time, the neon lamp fires thecontrolled rectifier 66, which in turn rapidly switches an effectiveshort circuit across the series circuit including the transformerwinding 72 and the capacitor 76. The capacitor 76 is discharged causinga damped oscillation to take place in the circuit. The discharge and theringing appear across the secondary winding 78 as. alternating highvoltage spikes, as illustrated in FIG. 6. The amplitude of the voltagespikes across the electrodes 26 and 28 is in the order of 13,000 voltsand are also rich in the Fourier spectrum with high frequencycomponents. The damped oscillation also appears across the anode ofcontrolled rectifier 66 causing the device to recover its blockingvoltage capability when the voltage reverses polarity.

The rate at which the controlled rectifier is rendered conductive andnon-conductive is dependent upon the combination of the size of theresistors 68 and 64, the threshold of the trigger device 70, theinductance of 72, the size of the capacitor 76 and the applied voltage.With the particular component values as disclosed in FIG. 3, thesemiconductor vibrator is activated (the controlled rectifier isrendered conductive) at a rate in the order of 2,000 cycles per second.However, the components can be in value to produce a wide range of lowfrequency cycles, such as, for example, 500 to 15,000 cycles. Theamplitude of the output pulses follow the amplitude of the rectified 120cycle pulses.

In the circuit diagram of FIG. 4, the supply and firing circuit ischanged from that of FIG. 3. For purposes of illustration, the samecomponents connected in the same circuit configuration in FIGS. 3 and 41 are designated by the same reference numbers. In FIG. 4, an inductor97 is connected to a bridge circuit 80' including the rectifiers 81, 82,83 and 84 and the combination is connected to the contacts of an ACpower plug 86. One-diagonal of the bridge circuit 80 is connected to areturn lead 88 and the other diagonal to one end of a ballast coil 90.The other end of the ballast coil 90 is connected in series with acurrent limiting resistor 92 to the anode of the controlled rectifier66. A phase shift firing circuit is connected between the resistor 92and the return line 88 including the series resistor 94 and thecapacitor 96. The junction of the resistor 94 and the capacitor 96 isconnected to the gate electrode of the controlled rectifier 66. Thereactor 90 and the resistor 90 may be omitted if the input circuit doesnot require limiting.

The circuit of FIG. 4 essentially functions in the same manner as thatof FIG. 3. The bridge circuit 80 rectifies the 60 cycle line voltage todevelop cycle rectified pulses. The controlled rectifier fires wheneverthe voltage charged across the capacitor 96 reaches the gate firinglevel, to develop alternating high voltage pulses coil. In all the laserpumping systems specified and as il I lustrated in FIGS. 5 and 6, thevoltage applied to the laser is alternating, irregular, ragged, hassteep slopes, and accordingly is rich in high frequency components thatprovide for the energization of the laser. Since the pumping systemdisclosed in the present application is not concerned with highamplitude peak operation of the laser, but rather essentially an outputthat appears to be continuous to an observer, there is no limitation onthe repetition rate or amplitudes in the pulses other than operating thelaser within its power capability. There is no need to shape theenergizing voltage pulses in any particular single form. The onlyrequirement is sufficient amplitude and high content of high frequencycomponents. The repetition rate of the pulses of the peak applied to theelectrodes of the laser should be at a sufficient rate so that theoutput from the laser appears substantially continuous to an observer.

The type of electrodes used with this particular system were found notto be critical. Copper wire, aluminum foil. aluminum backed nylon tape,and aluminum coated surfaces were all found to be useful. However, itwas found that the more intimate the contact between the electrodematerial and the gas, and the greater area of tube covered, the moreefficient the coupling. Accordingly, it can be said the larger thediameter of the tube, the more efficient is the coupling. However, sincethe diameter of the tube is a compromise between mechanical rigidityrequirements and maximum coupling, the best coupling in the particularlaser disclosed was achieved by making the side arm 20 larger indiameter than the main tube and as long as practical, and wrapping theelectrode 26 thereabout. It is likewise desirable to make the otherelectrode 28 as long as possible and to wrap it around the end of thetube 10 opposite the side arm.

What is claimed is: v

1. A power source for a gas laser having electrodes in communicationwith a lasable gas mixture, comprising:

a transformer including a primary winding and a secondary windingexhibiting a high step-up turns ratio between the primary winding andthe secondary winding, the secondary winding connected to theelectrodes;

input circuit means connected to a source of unidirectional energizingpotential and the primary winding for causing current flow through theprimary winding;

switching means including a pair of contacts movable relative to eachother, electromagnetically coupled to the primary winding for actuationof the contacts when the current flow through the primary windingreaches a predetermined level; and circuit means serially connecting themovable contacts between the input circuit means and the primary windingwhereby the contacts operating as a function of the current in theprimary winding periodically interrupt the current flow from the sourceto develop high voltage pulses across the externally disposed electrodesconnected to the secondary winding causing the gas mixture to lase.

2. A power source for a gas laser having electrodes in communicationwith a lasable gas mixture, comprising:

a transformer including a primary winding and a secondary windingexhibiting a high step-up turns ratio between the primary winding andthe secondary winding, the secondary winding connected to theelectrodes;

input circuit means connected between the primary winding and a sourceof energizing potential;

a capacitor coupled in series with the primary winda controlledrectifier having its anode and cathode connected across the seriescircuit including the capacitor and the primary winding; and

a trigger circuit coupled to the gate electrode of the controlledrectifier energized through the input circuit means by the source ofenergizing potential for periodically firing the controlled rectifierand thereby rapidly switching a low impedance path across the seriallyconnected capacitor and the primary winding at a low frequency rate forthe secondary winding to develop alternating irregular high voltagepulses having substantially high frequency components in the megacyclerange across the electrodes causing the gas mixture to lase.

3. The lasing apparatus as defined in claim 2, wherein:

the trigger circuit includes a neon lamp.

4. The lasing apparatus as defined in claim 2, wherein:

the trigger circuit includes a capacitor connected across the gate andcathode of the controlled rectifier.

5. In a gas laser having electrodes in communication with a lasable gasmixture for receiving electrical energy for exciting the gas mixture, animprovement for generating the electrical energy for exciting the gas 45mixture, the improvement comprising:

a step-up transformer including a primary winding for inductivelycoupling a secondary winding which is connected to the electrodes;source of unidirectional electrical energy connected in series with theprimary winding; and vibrator connected in parallel with the primarywinding of the step-up transformer and the energy source for providingalternating pulses of energy from the energy source to the primarywinding at a substantially low frequency to thereby induce in thesecondary winding high voltage alternating pulses which include highfrequencies in the megacycle range for exciting the gas mixture to lase.

6; The improvement for exciting the gas mixture as 60 defined in claim5, wherein:

the vibrator is electromagnetically coupled to the primary winding forproviding alternating pulses of electrical energy from the energy sourceto the primary winding at the substantially low frequency, in a range inthe order of 80 to 400 cycles per second.

7. [n a gas laser having electrodes in communication with a lasable gasmixture for receiving electrical energy for exciting the gas mixture, animprovement for generating the electrical energy for exciting the gasmixture, the improvement comprising:

step-up transformer including a primary winding for inductively couplinga secondary winding which is connected to the electrodes;

capacitor connected in series with the primary winding of the step-uptransformer;

source of unidirectional electrical energy connected in series with thecapacitor; and

switching circuit connected in parallel with both the energy source, andthe capacitor and the primary winding of the step-up transformer foraltemately providing a low impedance path across the capacitor forproviding alternating pulses of energy from the energyrsource to theprimary winding at a substantially low frequency to thereby induce inthe secondary winding high voltage alterhating pulses which include highfrequencies in the r'negacycle range for exciting the gas mixture tolase.

8. The improvement for exciting the gas mixture, as

defined in claim 7, wherein:

the switching circuit includes a semiconductor switching device and atrigger circuit for firing the semiconductor.

9. The improvement for exciting the gas mixture, as

10 defined in claim 8, wherein:

said semiconductor switching device is a controlled said source providesfull wave rectified electrical pulses; and

said trigger circuit fires said controlled rectifier cir-

2. A power source for a gas laser having electrodes in communicationwith a lasable gas mixture, comprising: a transformer including aprimary winding and a secondary winding exhibiting a high step-up turnsratio between the primary winding and the secondary winding, thesecondary winding connected to the electrodes; input circuit meansconnected between the primary winding and a source of energizingpotential; a capacitor coupled in series with the primary winding; acontrolled rectifier having its anode and cathode connected across theseries circuit including the capacitor and the primary winding; and atrigger circuit coupled to the gate electrode of the controlledrectifier energized through the input circuit means by the source ofenergizing potential for periodically firing the controlled rectifierand thereby rapidly switching a low impedance path across the seriallyconnected capacitor and the primary winding at a low frequency rate forthe secondary winding to develop alternating irregular high voltagepulses having substantially high frequency components in the megacyclerange across the electrodes causing the gas mixture to lase.
 3. Thelasing apparatus as defined in claim 2, wherein: the trigger circuitincludes a neon lamp.
 4. The lasing apparatus as defined in claim 2,wherein: the trigger circuit includes a capacitor connected across thegate and cathode of the controlled rectifier.
 5. In a gas laser havingelectrodes in communication with a lasable gas mixture for receivingelectrical energy for exciting the gas mixture, an improvement forgenerating the electrical energy for exciting the gas mixture, theimprovement comprising: a step-up transformer including a primarywinding for inductively coupling a secondary winding which is connectedto the electrodes; a source of unidirectional electrical energyconnected in series with the primary winding; and a vibrator connectedin parallel with the primary winding of the step-up transformer and theenergy source for providing alternating pulses of energy from the energysource to the primarY winding at a substantially low frequency tothereby induce in the secondary winding high voltage alternating pulseswhich include high frequencies in the megacycle range for exciting thegas mixture to lase.
 6. The improvement for exciting the gas mixture asdefined in claim 5, wherein: the vibrator is electromagnetically coupledto the primary winding for providing alternating pulses of electricalenergy from the energy source to the primary winding at thesubstantially low frequency, in a range in the order of 80 to 400 cyclesper second.
 7. In a gas laser having electrodes in communication with alasable gas mixture for receiving electrical energy for exciting the gasmixture, an improvement for generating the electrical energy forexciting the gas mixture, the improvement comprising: a step-uptransformer including a primary winding for inductively coupling asecondary winding which is connected to the electrodes; a capacitorconnected in series with the primary winding of the step-up transformer;a source of unidirectional electrical energy connected in series withthe capacitor; and a switching circuit connected in parallel with boththe energy source, and the capacitor and the primary winding of thestep-up transformer for alternately providing a low impedance pathacross the capacitor for providing alternating pulses of energy from theenergy source to the primary winding at a substantially low frequency tothereby induce in the secondary winding high voltage alternating pulseswhich include high frequencies in the megacycle range for exciting thegas mixture to lase.
 8. The improvement for exciting the gas mixture, asdefined in claim 7, wherein: the switching circuit includes asemiconductor switching device and a trigger circuit for firing thesemiconductor.
 9. The improvement for exciting the gas mixture, asdefined in claim 8, wherein: said semiconductor switching device is acontrolled rectifier; said source provides full wave rectifiedelectrical pulses; and said trigger circuit fires said controlledrectifier circuit one or more times for each of said rectified pulses.